Autosomal Recessive Polycystic Kidney Disease (ARPKD) is an inherited, multi-organ disorder that affects 1/20,000 children. The disease is characterized by both polycystic kidneys and congenital hepatic fibrosis (CHF). The kidney disease, characterized by enlarged kidneys with diffuse microscopic collecting duct cysts, is usually evident at birth. Kidney failure develops in 40-50% of affected children by age 10. ARPKD liver disease (CHF) is a biliary tract lesion characterized by both bile duct proliferation and dilatation as well periportal fibrosis. CHF is typically more slowly progressive and is clinically significant in 15-20% of patients. However, it is likely to become increasingly prevalent as more patients survive into adulthood. CHF can result in life-threatening complications and contributes to morbidity and mortality in ARPKD patients who have undergone kidney transplantation. Unfortunately, there are currently no established methods for monitoring kidney or liver disease progression in ARPKD. Traditional measures of kidney or biliary function (such as serum creatinine or bilirubin) may be normal and/or stable despite ongoing disease progression. And, unlike ADPKD, conventional diagnostic imaging techniques are also limited as kidney size may also be stable over time. Invasive kidney and liver biopsies are also not useful for longitudinal monitoring of ARPKD progression. The absence of quantifiable indicators of progression in this multi-organ disease not only limits our ability to assess kidney and/or liver disease progression, but also severely limits the ability to study therapeutic interventions. This is particularly problematic because several therapies have been shown to be effective in slowing kidney or liver disease progression in ARPKD animal models. The proposed studies will be conducted in the PCK rat model of ARPKD. The Specific Aims are: (1) to develop MRI imaging measures of cystic burden in ARPKD kidney disease progression; (2) to develop MRI imaging assessments of biliary expansion and periportal fibrosis in ARPKD liver disease progression; and (3) to test the applicability of longitudinal MRI assessments to monitor disease progression and response to therapy. The Diffusion and Magnetization Transfer MRI techniques developed in Aims 1 and 2 will be validated with histological measures of ARPKD kidney and liver disease, respectively. In Aim 3, these MRI techniques will be used longitudinally assess disease progression and response to novel therapies. These imaging studies are highly translatable and may provide important data for future imaging and therapeutic studies in ARPKD patients.